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数据加载中……

eclipse + JBoss 5 + EJB3开发指南(9):实现Entity Bean的多对多(many-to-many)映射

本文为原创,如需转载,请注明作者和出处,谢谢!

上一篇:eclipse + JBoss 5 + EJB3开发指南(8):实现Entity Bean的一对多(one-to-many)映射

    在EJB3中需要使用@ManyToMany对封装多对多关系的字段或getter方法进行注释。先看看下面的表:
图1  t_addresses表

      t_addresses表和t_customers表是多对多的关系。需要使用一个关联表来描述这种关系,关联表的结构如下图所示。
图2  t_customers_addresses
    在Customer类中定义一个Collection<Address>类型的字段(addresses),用于保存与该Customer对象相对应的多个Address对象,代码如下:
package entity;

import java.util.Collection;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.OneToMany;
import javax.persistence.OneToOne;
import javax.persistence.PrimaryKeyJoinColumn;
import javax.persistence.Table;

@Entity
@Table(name 
= "t_customers")
public class Customer
{
    
private int id;
    
private String name;
    
private Referee referee;
    
private Collection<Order> orders;

    
private Collection<Address> addresses;

    @ManyToMany(cascade = CascadeType.PERSIST, fetch = FetchType.LAZY)
    @JoinTable(name 
= "t_customers_addresses", joinColumns = @JoinColumn(name = "customer_id",
     referencedColumnName 
= "id"), inverseJoinColumns = @JoinColumn(name = "address_id", referencedColumnName = "id"))
    
public Collection<Address> getAddresses()
    {
        
return addresses;
    }

    ... ...

}

    @JoinTable注释用于指定连接表和t_customers及t_addresses表的连接字段关系。
    Address类的代码如下:

package entity;

import java.util.Collection;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
import javax.persistence.Table;
@Entity
@Table(name
="t_addresses")
public class Address
{
    
private int id;
    
private String addressLine;
    
private String country;
    
private String postCode;
    
private Collection<Customer> customers;
    @Id
    @GeneratedValue(strategy
=GenerationType.IDENTITY)
    
public int getId()
    {
        
return id;
    }
    
public void setId(int id)
    {
        
this.id = id;
    }
    
public String getAddressLine()
    {
        
return addressLine;
    }
    
public void setAddressLine(String addressLine)
    {
        
this.addressLine = addressLine;
    }
    
public String getCountry()
    {
        
return country;
    }
    
public void setCountry(String country)
    {
        
this.country = country;
    }
    
public String getPostCode()
    {
        
return postCode;
    }
    
public void setPostCode(String postCode)
    {
        
this.postCode = postCode;
    }
    @ManyToMany(mappedBy
="addresses")
    
public Collection<Customer> getCustomers()
    {
        
return customers;
    }
    
public void setCustomers(Collection<Customer> customers)
    {
        
this.customers = customers;
    }    
}

    由于是多对多的关系,因此,在Address类中需要定义一个Collection<Customer>类型的字段(customers)用来保存与该Address对象相对应的Customer对象。getCustomers方法也需要使用@ManyToMany进行注释。可以使用下面代码进行测试:
Customer customer = new Customer();
customer.setName(
"微软11");
List<Address> addresses = new ArrayList<Address>();
Address address 
= new entity.Address();
address.setAddressLine(
"address1");
address.setCountry(
"中国"); 
address.setPostCode(
"12345678");
addresses.add(address);
address 
= new entity.Address();
address.setAddressLine(
"address2");
address.setCountry(
"美国"); 
address.setPostCode(
"4321");
addresses.add(address);
customer.setAddresses(addresses);

em.persist(customer);

下一篇:eclipse + JBoss 5 + EJB3开发指南(9):实现Entity Bean的多对多(many-to-many)映射



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posted on 2009-06-03 16:15 银河使者 阅读(1663) 评论(0)  编辑  收藏 所属分类: java 原创ejb3JBoss


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